Clarification of sugar cane juice



Sept. 16, 1952 K. H. EAKIN 2,610,932

CLARIFICATION OF SUGAR CANE JUICE Filed July 27, 1950 10 11\ 12\ EVAPORATORS R OLARIFIER r HEATE 12 14 A BAGASSE v VACUUM 13 PUMP Q CI T0-TANK- 24/ HEATER STRAINERS \25 21 VACUUM-:- FILTER CAKE EVAPORATORS INVENTOI? 1 Keith H Eakin 32 p Patented sept. 16, 1952 CLARIFICATION OF SUGAR GANE JUICE Keith H. Eakin, ParkForest, Ill.,'assignor to The s De Laval Separator Company, New York, N. 1a,

a corporation of New Jersey I r g Application July 27, 1950, sea-1N0. 176,132 f The invention relates to the clarification of sugar cane juice and has particular reference to an improved method and system for this purpose.

The processing of cane sugar usually involves five stages, namely (1,) crushing of the cane and extraction of the juice, (2) defecation ,ofxthe juice, (3) clarification of the juice, ,(4) concentration of theclarified juice by evaporation, and $5,) crystallization of the sugar. Inthe first stage, theharvestedcane isshredded and crushed by passage through: the mill system, which may consistof several heavy crushing rollspreceded by the cane cutters and shredders. As the cane passes through the rolls, it-vis' crushed sothat the rawsugar juices are squeezed from it and drain through the bottom of each mill stand. Passingzcounterecurrently to the crushed cane, water is fedqto the lastmill and finally emerges from the first mill as the crude juice. The crushed, and extracted cane leaving the last mill is known as bagassej! which is used as fuel for the manufacture .of-by-products.

The second stage, or defecation, is a clarification process similar to that used in the beet sugar industry, in which milk of lime is added to the juice to precipitate insoluble impurities and at 6 Claims, (c1. rav -'5) i from the cane in the the same time to carry down a considerable amount of insoluble sediment in the subsequent clarification. The liming is generally performed inwlarge batch tanks which supply the juice to a continuous gravity'settler or .clarifier. In some cases, the :defecationstage also involves treatment of the juice with sulfur dioxide gas.

' In the third or clarification stage, the limed juice is heated to a temperature of 212-220 F. before it is fed-to the gravity settler. The fiow of the juice to the settler is maintained at such a rate thatmost of the suspended heavy solids are able to settle to. the bottom of thev settling tank as a mud, which is continually removed by slowly revolving rakes or plows. The gravity settler may be of any desired type, such asa Dorr clarifier. The clarified Juice, which usually represents 85%-90% of the volume of the crude juice, overflows into a launder or tray at the upper part. of the settling tank and passes to the evaporators for concentration. The sediment or mud, which is removed continuously from the bottom of-the'settling tank, usually constitutes 1 0%-15% of the crude juice volume and contains the lime salts precipitated from the crude juiceby the liming operation, a considerable excess of lime, shredded. fiber from the cane, and a considerable quantityofdirt and sandwashed mills. The total sediment in'this mud is generally about 20% insoluble material. [The fiuid portion of the mud, approximately contains considerable amounts of sugar in solution, and in'the interest of economy it is necessaryto reclaim as much of this sugar 'a'spo'ssible. 1

Heretoforait has been common practice to recla'im...sugarffrom' the mudin the gravity settler ;b'y. feeding the mud to a vacuum filter, after adding 'tothe mud a considerable amount of bagasseto act as a, filteraide The vacuum filterywhich may be'an Oliver filter, serves to remove the sediment as a cake, which is subsequently'washed to reclaim the soluble sugar. The filter consists of 'a large hollowrdrumrotating slowlyfon" a horizontal 'axis with the lower portion of the drum immersed in the feed liquid or mud from the clarifier. A partial vacuum is maintained on" 'the' submerged outersurfaces of the drum so as to suck relatively clear juice into the drum through the metal screen which forms its periphery. A cake of solids is thus built up on the-drum, and as the drum rotates the cake moves above the level of the feed liquid to a point in the-cycle of the drum rotation where it is continually washed with a spray of fresh water. Then, at a subsequent point in the cycle, a doctor. blade adjacent the surface of the drum scrapes ofi the cake, after which the cleaned drum. surface again moves down into the feed.

liquid to complete the cycle.

7 Since the smallest openingwhich is practical in the periphery of the filter drum isseldom less than 0.205, the filtrate from .the' vacuum filter, even when considerable amounts .of baga sse areused as a filter aid, generally contains from 2%-12% totalsuspended solids. One of the ,factors which prevents cleaner filtration is probably the cane waxpresent in the mud, which would act to seal completely the finer filter clothes which would be needed on the drum to clarify the filtrate more effectively. Because the filtrate, in any practical operation of the vacuum filter, contains a substantial quantity of mud, it is not possible to feed the filtrate directly into thev evaporating stage. Consequently, it has generally been necessary torecycle the filtrate from the vacuum filter to thepDoor clarifier or other gravity settling tank. This has been true under ordinary operating conditions, but, especially when the 'cane 'is harvested during wet weather; the amountof dirt in the raw cane may be solgreat 'thatjthe filtratefromlthe vacuum filter may contain as much as 12% suspended 3 solids. To return such a filtrate to the gravity settling tank or clarifier would overload the latter, with the result that the production line would soon have to be shut down. Accordingly, it is sometimes necessary to discard the filtrate, which means a loss of valuable sugar.

The present invention has for its principal object the provision of an improved process and system for recovering sugar from the mud in the gravity settler or clarifier. which overcomes the above-noted disadvantages in the prior treatment of this mud.

According to the invention, thesugar cane mud from the clarifier or gravity settling tank is delivered to a filter, preferably a vacuum filter of the Oliver type, where a portion of the sediment is removed as a filter cake from the liquid com-' ponent of the mud. This liquid component or filtrate, with the remaining mud, is then fed through a heater where it is heated to a temperature of at least 180 F. heated filtrate after beingv strained'toremovethe larger particles, is fed to a centrifugal separatorof the type in which the centrifugal bowl or locus of centrifugal force is provided at its outer periphe'ry'with outlets for discharging a sludge component. In the bowl, the filtrate is centrifugally separated into a'clarified juice component and a sludge "component, the latter being discharged through the peripheral outlets; and the clarified juice beingdischarged through a separate outlet. At *leastpart of the sludge componentidischarged'irom thefcentriiuge is recycled to: theffiltering operatiorr'ior further recovery of sugar from thesludge: .Breferably; the sludge, component"frfonr the "centrifuge is passed through a gravity. settling itankftor re'-' moval of the 'heaviertandmore erosive particles before" recycling of the sludge: The recycled sludge may be'delivered togthe filtering operation by mixingthe sludge 'Withjthe mud fed from the clarifier to the filter, or'by'spraying :the sludge uponthe filteric'ake; Thep'latten'may then be subjected to a'washing operation; along'with the recycled sludge, toremoveithe dissolved sugar. I have found that when the mudirom the clari: fier is treated in this 'mannerthe effluent or clari fied juice component from the centrifuge will'con: tain as'little as0.3% -0.6% solids; Atthe lower solids .content,. which is possible under more favorable conditions with respect to'the amount of dirt and sand in the raw cane-theefilueht-or clarified juice component from the centrifuge-is clean'enough to be fed directly to' theevaporator, thereby eliminating the necessityof recycling the juice to the clarifier. Thus,"the process produces a fresher'julce, simplifies the'operatlon, and'reduces operating costs. At the higher solids con-' tent, 0.4%0.6%, the efiluent from the centrifuge may be recycledto the gravity'settler; However, since the solids content is relativelyloweven in this case,the recycled filtrate 'does not overload the clarifier, and so it is not necessary to incur the loss of filtrate whichwould-result from the prior practices, especially during periods of wet weath- By recycling part ofthesludge component from the centrifuge to the inle't line of the centrifuge, it is possible tdregul'ate the concentra tion of the sludge component, :which may .contain upto %:soli'ds. For a'better understanding of the'invention, reference maybe had tothe accompanying drawing in which the'sin'gle illustration is a schematic view of a preferredisystem for practicing the invcntionm" or:

Referring to the drawing. the limed sugar juice from the defecating operation is delivered through a pipe Hi to a heater II, where it is heated to a temperature of about 212-220 F. From the heater II, the juice is fed continuously to a gravity settler or clarifier l2, which is preferably of the Dorr type, having revolving rakes or plows (not shown) for' continuously discharging the settled mud through a bottom outlet in. The clarified juice, approximately of the crude juice volume, is discharged from the upper portion of the clarifler [2 through launders or trays tolan outlet 12b, from which the clarified juice is delivered to the usual evaporators. The mud which'settles to the bottom of the clarifier i2 is discharged through outlet I2a to a feed tank I {where the 'mud is mixed with a considerable amount of fine bagasse fed from a pipe 14, the bagasse acting as a filter aid in the subsequent filtering operation. 7

The mud-is" fed continuouslyirom tank 13 through a pipe I5-t0' a vacuum filter f6, whichis preferably of the Oliver type. The filter comprises a container I! for receiving the mud, and a hollow drum l8 rotatable on ahorizontal axis 181:. The periphery of the drum is formed by a metal screen se that the drum is-perforate'd, and the drum rotates-slowly with its lower por'tionlmmersed in the mud iri container l1. Theinterior of the drum is connected-atthe axis [8a to a suction line l3, whereby a' partial vacuum is maintained on the submerged outer surfaces oi the drum so as tosuckrelatively'clear juice from container 11 through the perforatedperiphery of the drum and into the vacuum line I9. Consequently, a cake of solids accumulates on the outer surface of the drum. As the drum rotates, this cake-is moved above the level'of the mud in the container" I1 and carried around-to a doctor blade 20 pressing against the drum surface, so that the cake is scraped off the-drum and discharged from the filter as shown at ile-Theclean'ed drum surface then rolls downlinto themud in the container l1, and the cycle is repeated. Prior to the scrapingioperation by thebladesn, the filter'cake is preferably washed with ia spray of fresh water delivered'throughlaapipe 16d.

The filtrate from the filter I 6 is sucked through the pipe line" Hi to a closed tank 22 which is maintained under a partiarvacuum by a vacuum pump 23. The filtrate passes from the tank through a heater 24 where it is heated to a. temperature of at least. The heatedwfiltrate is then passed through twin strainers :25 to-Lremove the larger particles frornlthe'flltrate, and from the strainers 255the filtrate is delivered toa feed tank 26. The latter' is connected through a pipe 26a to the inlet of :a centrifuge 21 having a sludge outlet 21a: anjd-za: clarified .julce outlet 21b. The centrifuge 21 is preferablyof the De Laval AC-VO- type irrrwhich the centrifuge bowl is provided with-peripheral'outlets for th'e'sludge anda sepa rate outlet located nearer thebowl axiaior discharging the clarified Juice.

The separated. sludge component discharged from" the centrifuge outlet 2'ldls deliveredto a sludge collecting tank (28." The tank 28 functions as alfeed tank for pumps 29 and 32 in pipe lines 30 and-3l,' respectively, and is preferably fitted with a verticalpartitiori-nal In the tank 28, the heavier and more erosive" particles of the sludge settle out; the'li'ghter' particles passing over the partitioni Zliw to a sub-compartment 28b" ofthe tank: ,Ktcdeast :part iot f'tlie discharged sludge vconwonentu's ,fed continuously iron: the c'om-' partment 28b by a centrifugal pump 29 to a pipe line 39, through which the sludge component is recycled to the filtering operation, as by returning the sludge to the feed tank It. Alternatively, the recycling of the sludge component may be effected by feeding it from pipe 30 through a branch pipe 30a which sprays it upon the filter cake on the drum i8.

Part of the discharged sludge component from the centrifuge is preferably fed from the sludge collecting tank 28 through a pipe 3! to a centrifugal pump 32 which feeds the sludge through twin filters 33 and pipe 34 to the centrifuge feed tank 25. In this way, it is possible to increase the concentration of the sludge component discharged from the centrifuge outlet 21a, and by regulating the rate of recycling to the feed tank 25, the concentration of the sludge may be adjusted.

The effluent or clarified juice separated in the centrifuge 27 is discharged through outlet 21b to a pipe 35. From the latter, the clarified juice component may be fed to the evaporatcrs. However, in cases where the mud from the clarifier l2 contains excessive quantities of sand and dirt, I prefer to recycle the sludge component directly from the centrifuge outlet 21b through pipes 35 and 35a to the clarifier l2.

The practice of the present invention enables a practically complete recovery of sugar from the mud in the clarifier or gravity-settling zone i2, and yet the added load, if any, on the gravitysettling zone I 2 is inconsequential, even under the most adverse conditions with respect to the amount of sand and dirt in the raw cane. Sludge from the centrifuge 27 is subjected to a further sugar-recovery operation as a result of its recirculation to the filter l6, either by being mixed directly with the feed to the filter receptacle IT or by being delivered directly to the filter cake. The sludge combined with the filter cake which is discharged at 2! may be further processed for recovery of remaining sugar. Since it is not necessary to effect a fine filtering of the material at the filter it, the latter may be operated at relatively high capacity so as to handle the increased load represented by the sludge recycled to the filter feed tank !3.

I have found that the heating of the filtrate prior to its introduction into the centrifuge 21 is an important factor contributing to the successful operation of the new process and system. Preferably, the filtrate is heated in the heater 24 to a temperature of 200 F. or higher. Unless the filtrate is heated to a temperature of at least 180 F. prior to the centrifuging, the centrifugal bowl will become clogged or plugged after a relatively short period of operation, with the result that the recovery of sugar is impaired. This is evidently due to the presence of Wax in the filtrate, and by centrifuging the filtrate at a temperature of about 200 F. or more, which is somewhat above the melting point of the wax, the centrifugal separation is greatly facilitated.

It will be apparent that the sludge settlin tank 28, pump 29 and pipe 30 constitute a means for recycling at least part of the sludge component from centrifuge 21 to the filtering operation at filter It, by Way of either the feed tank H5 or the branch pipe 39a, or both. In the sludge tank 28, the heavier and larger particles are removed from the sludge, so that the system recycles to the centrifuge only the smaller particles of sludge, which have less of an erosive action .on the centrifugal bowl and do not clog the outletsfor the sludge.

I claim:

1. In the clarification of sugar cane juice containing suspended heavy solids, by feeding the juice to a gravity-settling zone where the solids settle to the bottom as amud, separately discharging the clarified juice and the mud from the upper and lower regions, respectively, of said zone, and subjecting the mud from said zone to a filtering operation to recover juice therefrom as a filtrate, leaving a filter cake of mud, the improvement which comprises straining said filtrate, feeding the strained filtrate at a temperature of at least 180 F. to a locus of centrifugal force and there separating it centrifugally into; a clarified juice component and a sludge comfiltering operation by spraying the sludge component on said filter cake.

4. The improvement according to claim 1, in

which said sludge component is recycled to the filtering operation by spraying the sludge component on said filter cake, the improvement comprising also the step of washing the filter cake together with said sludge component to recover the soluble sugar therein.

5. The improvement according to claim 1, comprising also the-step of feeding part of the sludge component to the centrifugal locus along with said strained filtrate, to increase the concentration of said sludge component.

6. The improvement according to claim' 1, comprising also the step of recycling said clarified juice component to said gravity-settling zone.

KEITH H. EAIQN.

No references cited. 

1. IN THE CLARIFICATION OF SUGAR CANE JUICE CONTAINING SUSPENDED HEAVY SOLIDS, BY FEEDING THE JUICE TO A GRAVITY-SETTLING ZONE WHERE THE SOLIDS SETTLE TO THE BOTTOM AS A MUD, SEPARATELY DISCHARGING THE CLARIFIED JUICE AND THE MUD FROM THE UPPER AND LOWER REGIONS, RESPECTIVELY, OF SAID ZONE, AND SUBJECTING THE MUD FROM SAID ZONE TO A FILTERING OPERATION TO RECOVER JUICE THEREFROM AS A FILTRATE, LEAVING A FILTER CAKE OF MUD, THE IMPROVEMENT WHICH COMPRISES STRAINING SAID FILTRATE, FEEDING THE STRAINED FILTRATE AT A TEMPERATURE OF AT LEAST 180* F. TO A LOCUS OF CENTRIFUGAL FORCE AND THERE SEPARATING IT CENTRIFUGALLY INTO A CLARIFIED JUICE COMPONENT AND A SLUDGE COMPONENT, SEPARATELY DISCHARGING SAID COMPONENTS FROM SAID LOCUS, AND RECYCLING AT LEAST PART OF THE SLUDGE COMPONENT FROM SAID LOCUS TO THE FILTERING OPERATION TO RECOVER SUGAR FROM THE SLUDGE. 